I've never had a problem so as long as the content is pretty much all unique there should be no issues. Supposedly it's possible to run duplicate content targeted at different markets and still be ok i.e. the same site for .com and .co.uk

I think it depends also on the backlink profiles of the particular sites. If each site has many independent inbounds from unrelated sources, then they might do OK with the same IP or related hosting.

If the sites are related in market area, though, and they've historically been promoted in tandem so that they have similar backlink profiles, then I would definitely keep hosting separate.

This is based on several experiences from some years back... with "sister" sites for different brick and mortar businesses owned by the same umbrella companies. The difference in separating hosting was so dramatic that I've never seen reason to experiment again.